The present invention relates to composite noble metal wires having an electrically-conductive, non-noble metal-containing core clad with a noble metal annulus. In particular, the present invention relates to composite wires formed by co-extrusion of the non-noble metal-containing core material and the noble metal. The present invention further relates to methods of forming composite wires in which a core containing a non-noble metal is clad with a noble metal annulus by co-extrusion of the non-noble metal core material with noble metal.
Advances in semiconductor packaging towards finer pitches, longer spans, and lower packaging costs are not adequately met by present gold bonding wire technology. The wire modulus, cost and strength requirements dictate the use of a more complex wire material than universally accepted 4N gold alloy.
Future bonding wire may need to conform to requirements of approximately 50 microns pitch at an approximately 5000 micron span, at a wire diameter of 20 microns. Bonding wire sag and sway are concerns in such a configuration. The sway deflection of one wire relative to another must be limited to about 30 microns. The relative strain required to cause a short between adjacent wires is less than 0.005%, which is an elastic strain. 4N gold alloys have an increased modulus over pure gold, but gold-based bonding wire alloys are not expected to have a sufficient modulus for such future requirements.
Copper is an ideal bonding wire in terms of modulus, resistivity, density and cost. However, oxidation concerns and higher bonding costs have prevented copper from becoming a common bonding wire material.
U.S. Pat. No. 5,097,100 discloses a noble metal-plated copper wire. A drawn copper wire having a diameter of from about 44 to 56 microns is electrolytically plated with gold, the surface of which may be cold-drawn to harden the gold layer.
However, it is not possible to uniformly plate a gold layer of adequate purity at a reasonable cost. The gold may not adequately adhere to a copper core by following the disclosure of U.S. Pat. No. 5,097,100. The other metal coating deposition techniques disclosed by this patent, including electroless plating, vapor deposition, sputtering, dipping, and the like are problematic for the same reasons. Furthermore, none of these techniques can coat a copper wire core with a 4N gold alloy bonding wire sheath.
While U.S. Pat. No. 5,097,100 discloses that the copper and gold may be co-drawn, there is no teaching, let alone a working example, of how this may be accomplished with micron dimensioned wires. There remains a need for a composite gold-clad copper wire that is capable of meeting the anticipated future performance requirements of the semiconductor industry at a reasonable cost.
This need is met by the present invention. It has now been discovered that composite wire having a non-noble metal core of consistent diameter with a noble metal layer of uniform thickness firmly adhered thereto may be economically produced by forming the noble metal layer on a non-noble metal core before the wire is drawn.
Therefore, according to one aspect of the present invention, a composite wire is provided consisting essentially of a wire core containing an electrically-conductive non-noble metal, and a noble metal annulus metallurgically bonded to the wire core.
Copper is the preferred non-noble metal, and a wire core consisting essentially of copper is most preferred. The noble metal is preferably gold, and more preferably gold having a purity greater than 90%. The purity is preferably greater than 99% and even more preferably greater than 99.99%. Preferably, the gold is a gold alloy in which the gold is doped to obtain sound deformation of the gold/copper composite as it is drawn, and good bonding properties for the composite wire, such as, for example, gold doped with less than 30 ppm of calcium, less than 20 ppm of beryllium, and less than 50 ppm of other elements. A particularly preferred alloy is 4N gold.
The present invention provides a method by which a non-noble metal wire may first be coated with a noble metal and then drawn to micron dimensions rather than attempting to form a layer of noble metal on a micron-dimensioned wire. Therefore, according to another aspect of the present invention, a method is provided for forming a micron-dimensioned composite wire consisting essentially of a conductive wire core containing a non-noble metal and a noble metal annulus metallurgically bonded thereto, wherein the method includes:
providing a first composite wire having a diameter between about 0.5 and about 5 millimeters, wherein the first composite wire consists essentially of a core containing a non-noble metal, and a noble metal annulus metallurgically bonded to the core; and
drawing the first composite wire to form a second composite wire having a diameter between about 15 and about 75 microns, so that the core fraction measured by cross-sectional area of the second composite wire is essentially the same as the core fraction of the first composite wire.
The first composite wire is drawn from a composite rod produced by co-extrusion of a noble metal billet having a non-noble metal core material thereby metallurgically bonding the noble metal and core metal layers. The composite wire having a diameter of 20 microns is drawn from a composite wire having millimeter dimensions, which in turn is formed from a composite cylindrical rod formed by extrusion of a composite billet, with the relative cross-section of the composite core and noble metal layer remaining unchanged from the billet to the rod to the wire. This permits direct control of the core fraction of the nominally 20 micron diameter composite wire to a degree heretofore unknown. Therefore, according to another aspect of the present invention, a composite wire is provided, having a micron-dimensioned diameter, prepared by the method of the present invention.
In other words, the desired core fraction, for example, for a 20 micron diameter composite wire, is produced by a composite billet having the same relative fraction of core material. By constructing a billet having the fraction of core material desired for the composite wire end product, a micron dimensional composite wire is obtained having the desired fraction of core material.
The composite wires of the present invention possess the desired modulus, strength and conductivities required for semiconductor packaging, and at the same time provide a cost advantage. Therefore, according to another aspect of the present invention, a semiconductor package is provided having at least one lead bonded to the second composite wire of the present invention. Composite wires having diameters as small as 25 microns have been wedge-bonded without disrupting the continuity of the noble metal outer layer, which is necessary in order to avoid oxidation of the non-noble metal core.
The composite wires of the present invention may be employed in other end-use applications for fine wire. Such applications include, but are not limited to, wires or cabled wires for jewelry, cathodic protection, or for harsh environments. The foregoing and other objects, features, and advantages of the present invention are more readily apparent from the detailed description of the preferred embodiments set forth below, taken in conjunction with the accompanying drawings.